The present invention relates to photomultiplier tubes and, more particularly, to tubes having a plurality of electrodes, including secondary dynodes and an anode, at least one of the electrodes having reference means formed therein and a mesh member attached thereto. The mesh member includes locating means for aligning the mesh member with the reference means. The electrodes each have support shoulders formed therein which provide stop locations adjacent to the support plates which carry the electrodes to prevent the mesh members from contacting the support plates.
In the manufacturing of a high volume photomultiplier tube, conflicting objectives are frequently encountered. The tube must be designed to provide accurate and reproducible measurement of the phenomenon being observed, and yet the tube must be inexpensive to produce. The former objectives demand that the position of each element of the tube be fixed accurately with respect to the other tube elements; however, this must be done without the use of a large number of expensive precision parts. In order to achieve the latter objective, the tube must be easy to assemble in order to eliminate the need for difficult, time consuming assembly techniques.
U.S. Pat. No. 4,125,793, issued to Timan on Nov. 14, 1978, discloses a photomultiplier tube having a box and grid (i.e., dynode mesh) structure with a floating anode that is not supported by the insulating side supports which rigidly hold the dynodes in fixed spatial relation. The purpose of the floating anode is to isolate the anode from the side supports which are disclosed to carry ohmic leakage or leakage currents. There is no disclosure concerning the manner in which the dynodes are affixed to the side supports or of the need for accurately locating and aligning the mesh with the dynodes to prevent deformation by the side supports.
It has been found that in an electron multiplier structure, such as that disclosed in the Timan patent, in which a dynode mesh extends across the dynodes, improper alignment of the mesh permits the mesh to contact the insulating side supports which frequently deform the dynode mesh to the point where the mesh alters the electrostatic field of the dynode causing a decrease in tube performance. Severe distortion of the mesh may even result in the mesh contacting an adjacent electrode, causing an electrical short.
It is therefore desirable to provide a means for accurately aligning the dynode mesh with the dynodes and to prevent the mesh from contacting the insulating supports which support the dynodes. The means must be cost effective yet simple to use in order to eliminate the need for complex and time consuming assembly techniques.